The present invention is directed to improving the ability to methods of modulating the action of the phytohormone abscisic acid (ABA) in plants. Modulating ABA activity in plants can be used, for example to confer drought tolerance on plants.
The phytohormone ABA regulates many agriculturally important stress and developmental responses throughout the life cycle of plants. In seeds, ABA is responsible for the acquisition of nutritive reserves, desiccation tolerance, maturation and dormancy (M. Koornneef et al., Plant Physiol. Biochem., 36:83 (1998); J. Leung & J. Giraudat, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 49:199 (1998)). During vegetative growth, ABA is a central internal signal that triggers plant responses to various adverse environmental conditions including drought, salt stress and cold (M. Koornneef et al., Plant Physiol. Biochem., 36:83 (1998); J. Leung & J. Giraudat, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 49:199 (1998)). A rapid response mediated by ABA is stomatal closure in response to drought (J. Leung & J. Giraudat, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 49:199 (1998); E. A. C. MacRobbie, Philos. Trans. R Soc. Lond. B Biol. Sci., 353:1475 (1998); J. M. Ward et al., Plant Cell, 7:833 (1995)). Stomata on the leaf surface are formed by pairs of guard cells whose turgor regulates stomatal pore apertures (E. A. C. MacRobbie, Philos. Trans. R Soc. Lond. B Biol. Sci., 353:1475 (1998); J. M. Ward et al., Plant Cell, 7:833 (1995)). ABA induces stomatal closure by triggering cytosolic calcium ([Ca2+]cyt) increases which regulate ion channels in guard cells (E. A. C. MacRobbie, Philos. Trans. R Soc. Lond. B Biol. Sci., 353:1475 (1998); J. M. Ward et al., Plant Cell, 7:833 (1995)). This response is vital for plants to limit transpirational water loss during periods of drought. Guard cells provide a well-suited system to characterize genes that affect early ABA signal transduction (F. Amstrong et al., Proc. Natl. Acad. Sci. U.S.A. 92:9520 (1995); Z.-M. Pei et al., Plant Cell, 9:409 (1997); J. Li et al., Science, 287:300 (2000)).
Two protein phosphatase mutations (abi1-1 and abi2-1) and a protein kinase mutant (aapk) that dominantly disrupt early events in ABA signaling (J. Leung & J. Giraudat, Annu. Rev. Plant. Physiol. Plant. Mol. Biol., 49:199 (1998); F. Amstrong et al., Proc. Natl. Acad. Sci. U.S.A., 92:9520 (1995); Z.-M. Pei et al., Plant Cell. 9:409 (1997); J. Li et al., Science, 287:300 (2000); K. Meyer et al., Science, 264:1452 (1994); J. Leung et al., Science, 264:1448 (1994)) and a recessive farnesyltransferase β subunit (era1-2) mutation that enhances early ABA signaling (S. Cutler et al., Science, 273:1239 (1996); Z.-M. Pei et al., Science, 282:287 (1998)) have been identified.
Identification of new ways of controlling ABA signal transduction would be desirable. Such methods would be particularly useful, for example, in controlling guard cell turgor and thus transpiration in plants. Such method would be particularly useful to limit transpirational water loss during periods of drought and thus render plants more drought tolerant. The present invention addresses these and other needs.